1. Field of the Invention
This invention relates in general to magnetic storage systems, and more particularly to a method for improving hard bias properties of layers of a magnetoresistance sensor.
2. Description of Related Art
Magnetic recording is a key and invaluable segment of the information-processing industry. While the basic principles are one hundred years old for early tape devices, and over forty years old for magnetic hard disk drives, an influx of technical innovations continues to extend the storage capacity and performance of magnetic recording products. For hard disk drives, the areal density on the magnetic medium has increased by a factor of millions since the first disk drive was applied to data storage. Such increases are based on corresponding improvements in heads, media, drive electronics, and mechanics.
Magnetic recording heads have been considered the most significant factor in areal-density growth. The ability of the magnetic recording heads to both write and subsequently read magnetically recorded data from the medium at data densities well into the gigabits per square inch (Gbits/in2) range gives hard disk drives the power to remain the dominant storage device for many years to come.
Important components of computing platforms are mass storage devices including magnetic disk and magnetic tape drives, where magnetic tape drives are popular, for example, in data backup applications. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk.
Read and write heads are directly mounted on a slider that has an Air-Bearing Surface (ABS) between the slider and the rotating disk. The suspension arm biases the slider into contact with the surface of the magnetic disk when the magnetic disk is not rotating. However, when the magnetic disk rotates, air is swirled by the rotating disk adjacent to the ABS causing the slider to ride on a cushion of air just above the surface of the rotating magnetic disk.
The write and read heads are employed for writing magnetic data to and reading magnetic data from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
A magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a sensing element as a function of the strength and direction of magnetic flux being sensed by the sensing element. Conventional MR sensors, such as those used as MR read heads for reading data in magnetic recording disk and tape drives, operate on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy. A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the magnetic disk in a magnetic disk drive, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element. This change in resistance may be used to detect magnetic transitions recorded on the recording media.
In the past several years, prospects of increased storage capacity have been made possible by the discovery and development of sensors based on the giant magnetoresistance (GMR) effect, also known as the spin-valve effect. In a spin valve sensor, the GMR effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium, or signal field, causes a change in the direction of magnetization of the free layer, which in turn causes a change in the resistance of the spin valve sensor and a corresponding change in the sensed current or voltage.
Magnetic sensors utilizing the GMR effect are found in mass storage devices such as, for example, magnetic disk and tape drives and are frequently referred to as spin-valve sensors. The spin-valve sensors are divided into two main categories, the Anti-FerroMagnetically (AFM) pinned spin valve and the self-pinned spin valve. A spin valve includes a pinned layer, a spacer and a free layer. The magnetization of the free layer is free to rotate in response to the presence of external magnetic fields. In an AFM pinned spin valve, the pinned layer has its magnetic moment pinned by a pinning layer. In the self-pinned spin valve, the magnetic moment of the pinned layer is pinned in the fabrication process.
The magnetic moment of the pinned layer may be pinned via magnetostriction phenomenon and stress anisotropy. Magnetostriction is the phenomenon in which a magnetic material changes its size depending on its state of magnetization. External mechanical stress may also contribute to the state of the magnetic moment. For example, a positive magnetostriction and compressive stress may be used to pin the pinned layer with the desired magnetic moment orientation. The self-pinned layer may be formed of a single layer of a single material or may be a composite layer structure of multiple materials. It is noteworthy that a self-pinned spin valve requires no additional external layers formed adjacent to the pinned layer to maintain a desired magnetic orientation of the pinned layer and, therefore, is considered to be an improvement over the anti-ferromagnetically pinned spin valve.
In the construction of a sensor using the GMR effect, a hard magnetic bias structure may be used to suppress the domain walls movement of the free layer to provide a noise-free reproducing waveform. This is accomplished by depositing hard magnetic thin films on both sides of the spin valve layers. A seedlayer structure is typically used to promote the texture of the hard bias films. Chromium is often used as a hard bias seedlayer whereupon the hard bias layers are grown. However, properties of the hard bias layers degrade significantly when deposited on spin valve layers. In order to make better junctions, partially milled sensor structures down to platinum manganese (PtMn) or other spin valve layers have been considered. The properties of the hard bias layer, however, are degraded significantly on PtMn or on other spin valve layers when deposited using the standard chromium (Cr) seedlayer.
It can be seen therefore, that there is a need for a method for improving hard bias properties of layers of a magnetoresistance sensor.